The erection of structures on soils with a low load-bearing capacity, such as clays and silts, requires either the transference of the load to more stable strata below or, in case such strata exist at too great a depth, the distribution of the load over a large volume of the soil in width as well as in depth, with the aim of reducing the specific load in all points of the soil and of preventing a detrimental settling of the foundation. In both cases the general practice is to drive or to cast into the soil piles of suitable dimensions and in a number sufficient for transferring the load of the structure to the soil.
Pile material -- which formerly was exclusively wood -- is today mostly concrete or corrosion-protected steel. Piles are either driven into the soil by pile hammers, or are cast in situ, in the form of cased or uncased concrete piles, in predetermined grouping and spacing, in a vertical and/or an inclined direction in order to take up vertical as well as horizontal loads.
Piles are effective in one of four ways: -- the first, by transferring the load through soft upper strata to the end bearing on a hard substratum; the second, as friction piles in their lower portions, transferring the load through soft upper strata into stiffer strata below; the third, as pure friction piles over their full length; and the fourth which is met with occassionally, by compacting the soil.
With friction piles the load is taken up by the soil adjacent to the pile surface and distributed over a larger volume of soil. Paradoxically, the pile surface should be smooth while the pile is being driven into the soil, in order to offer a minimum of resistance to the driving force; but it should be as rough as possible when the pile is in place, so as to present a maximum friction coefficient between surface and soil.
Cast concrete piles generally possess a smooth surface, whether they are cast in steel tube casings or uncased in clay or silt. The result is that for a certain load to be supported and to be transferred to the soil a smooth pile of considerably greater length and diameter is required than would be necessary with a pile having a very rough surface.
With increasing spans, weight and height of building structures the loading of the supporting soil naturally increases and requires a larger number of piles of great length and diameter, especially in all those cases in which they act as friction piles, whether over their entire length or over only part of it. Piles and pile-boring and driving equipment together constitute a large item of the total building cost.